Remotely controlled stimulator



REMOTELY CONTROLLED S T IMULATOR Filed May 2, 1962 2 Sheets-Sheet l |NVENTOR= HAROLD WARNER AGENT April 20, 1965 H. WARNER 3yl79p89fi REMOTELY CONTROLLED STIMULATOR Filed May 2, 1962 2 Sheets-Sheet 2 INVENTOR= HAROLD WARNER AGENT United States Patent REMGTELY CGNTRQ'LLED STKMULATQR Harold Warner, Norristowu, Pen, assignor to General Electric Qornpany, a corporation of New Yorlr Filed May 2, 1962, der. No. 191,791 2 Qlaims. (Cl. 325-32d) This invention pertains to surgical instruments and more particularly to a remotely controlled electrical stimulator which may be attached for long periods of time to a subject animal.

In the investigation of animal physiology and behavior, it is frequently desirable to stimulate some particular part of the animals nervous system in a manner which is easily controlled and accurately reproducible with respect to duration and intensity of the stimulation. For most purposes, electrical stimulation is preferred because its duration can easily be controlled and its intensity can easily be measured by any of the numerous devices for measuring current. Formany purposes, it is desirable that the stimulation be applied to the subject without altering its natural behavior pattern. To tms end, the stimulator should be small and light so that it can be attached directly to the animal without inconveniencing it and the stimulator should be remotely controlled. It is also desirable that the power requirements for the stimulator be as low as possible in order that the fredom which remote control permits may not be greatly impaired either by the necessity of approaching the animal fre quently for the purpose of providing new power sources or of burdening the animal by wei hting it down with heavy batteries. Finally, it is desirable that the stimulus, while it is predictable, be capable of being varied without approach to the animal.

In order to achieve constancy of current despite variations in the contact resistance between the animal and the connections to it, it is very advantageous to provide a current regulator. The existence of a device for main taining the current constant despite variations in the contact resistance does not necessarily preclude the possibility of altering the current by remote control, but I have not made provision in this device for doing so since this would necessarily involve the addition of some components with consequent addition of weight. I have, however, provided one degree of freedom in controlling the stimulus by making it possible to vary the duration of the constant current stimulus over a very wide range. It is known in the prior art to attempt to achieve some of the desirable characteristics enumerated by providing a radio receiver which triggers a pulse generator to produce a stimulus of approximately constant voltage, and of a substantially constant duration determined by the circuitry. This has the disadvantage that the provision of a constant voltage stimulus does not insure constant current; and that, in addition to the uncontrollable variations in stimulating current thus produced, the prior art device pro vides a stimulus duration which is fixed by the pulse generator and which, therefore, can be altered only by approaching the animal.

It is desirable for some purposes that the interval between stimuli be quite long, and for this reason the current drain of the device should be as low as possible when no stimulus is being applied. I have employed complementary transistors in my circuitry so that when no signal is being received by the receiver, all of the transistors are 0 "ice substantially cut oil. In order to eliminate the necessity of providing a separate antenna on the experimental subject, which antenna would be fragile, and impede the animals movement, and would cause it to appear strange to other animals (which might be undesirable for some tests), I employ the subject animal itself as the antenna, using the same contacts which are used to administer the stimulus. it is possible by my device to stimulate an animal controllably and accurately by the use of a device sufiiciently small and having a sufficiently low current drain so that it is quite reasonable to power such an animal with accumulators which are recharged by solar energy, assuming that the animal is of such sort that it is likely in its more or less normal pursuits to spend appreciable time in the sun.

For the better understanding of my invention, I have provided figures of drawing in which:

FTG. 1 represents a schematic diagram of an embodiment of my invention;

FIG. 2 represents a simplified schematic diagram of the current regulator of FIG. 1; and

FIG. 3 represents the general physical appearance of this embodiment as constructed to be applied to an experimental animal.

In FIG. 1, a coupling capacitor ltl is represented connected to a tuned circuit consisting of an inductor l2 and tuning capacitor 14 across which there is developed a radio frequency potential which is rectified by diode 1'6 and produces a drop across load resistor 18, which is bypassed for carrier-frequency signals by capacitor Zil. In operation, the potential developed across resistor 18 remains as long as a carrier-frequency signal is applied through capacitor ll Since it is not necessary experimentally to continue the stimulation of the subject for an extremely long period, the potential developed across resistor lid and capacitor Ztl by the rectification of the input signal is of limited duration. It may, therefore, be applied through coupling capacitor 22 to the base of transistor 2d. The rest potential of the base of transistor 24 is determined by the relative values of resistors 26 and 23 which are connected across a portion of battery 3%, that portion of the battery being bypassed by capacitor 32. The emitter of transistor 24 is fed through resistor 34 and its collector load is provided by resistor 36. The signal potentials appearing across resistor 36 are applied through blocking capacitor 38 to the base of transistor 4-9. The rest potential of the base of transistor 40 is determined by the values of resistors 42 and 44 which are connected across a portion of battery 30. The emitter of transistor ill is fed through resistor 46; and the collector of transistor 4-5) is furnished a load by resistor 48. The signal potentials appearing at the collector of transistor do are applied via capacitor St to the junction point of resistors 52 and 54, and thus to the base of transistor 56 whose collector is connected directly to the base of transistor 58. It will be observed that diode 16 is so poled that the potential developed across resistor 18 and applied through capacitor 22 is negative and tends to turn transistor 24- to the on state. This causes the collector of transistor 24 to become more positive and thus to apply through capacitor 38 a pulse which turns transistor 40 onv In consequence of this effect, the collector of transistor becomes more negative and applies a negitive pulse to the base of transistor 56. The collector of transistor 56 then becomes more positive and causes the base of transistor 58 to become more positive and thus causes transistor 58 to conduct. It is thus evident that a regular alternation of PNP and NPN transistors makes it possible to design an amplifier circuit all of whose transistors will be in the olf condition in the absence of an input signal and will draw a current only when a signal is applied and they all become conducting. This is distinctly beneficial in a piece of apparatus which is to o erate from batteries and will normally have a low duty cycle. The output of transistor 58, which is connected as an emitter follower and serves as a switch to connect the positive terminal of 30 to the regulator part of the circuit, is applied to a series of resistors 62, 64 and 66. Two of these, 62 and 64, are shunted by a Zener diode 68, which maintains approximately constant potential across resistors 62 and 64 by drawing the appropriate current through 66. A selected fraction of the potential appearing across resistor 62 is applied, as indicated, by a sliding contact to the base of transistor 69. Transistor 69 has in its emitter circuit a resistor 70 which is shunted by a monitoring jack 72 which may be used to connect a meter across resistor 70'for the purpose of adjusting the device. The emitter of transistor 69 is connected also to a radio frequency choke 74. The collector of transistor 69 is connected through a load resistor to the negative battery terminal, and the collector is also connected through resistor 78 to the base of a transistor 80, whose emitter is connected to the negative battery terminal and whose collector is connected to a tap switch 82, which is represented as being connected to a number of different terminal pins in a plug 83 which is used to connect the device to the actual electrode mount applied to the experimental subject. Terminal 84 of this plug 83 is connected as represented to radio frequency choke 74 and to corresponding condenser 10. The animal is connected by an electrode through terminal 84 so that the stimulating current returns through inductor 74 to transistor 69; and, also, carrier frequency signals picked up by the subject, acting as an antenna, are connected through capacitor to the input circuit of the diode receiver. Capacitor 10 blocks out the stimulating current and inductor 74 prevents the short circuiting of the carrier frequency signal.

Values of circuit parameters actually employed in one embodiment are shown in Table I. Type designations of semiconductors are those of the Electronic Industries Association, corresponding to characteristics made public by the said association, and are not the designations of a particular manufacturer, but correspond to industry standards.

FIG. 2 represents the portions of FIG. 1 which constitute the regulator circuit, omitting jack 72, inductor 74 and switch 82 and plug 83, these not being essential to a discussion of the operation of the regulator, and including a resistor 86, which represents the load resistance furnished by the subject animal. For convenience of reference, terminals 88 and 90 are indicated as reference points at which a potential is applied, 88 being positive and 90 being the corresponding negative reference.

The value of resistor 66 is so chosen that, when potential from battery is applied to terminals 88 and 90, the parallel combination of Zener diode 68 and resistors 62 and 64 in series will draw a current which will permit diode 68 to operate at the voltage which it tends to preserve as a stable reference value. Slight variations in the potential applied between terminals 88 and 90 will cause the Zener diode 68 to draw slightly more or less current, as may be required, to vary the drop across resistor 66 so that the drop across resistors 62 and 64 will remain substantially constant. Resistor 64 is fixed, and 62 is actually a potentiometer whose movable contact is connected to the base of transistor 69. The sole reason, of course, for having resistor 64 in series with 62 is to cause the potential drop across 62 to include only the desired range of voltages to be applied to the base of 69;

4 since the drop across diode 68 exceeds this desired range, resistor 64 is inserted to absorb the ditference between the voltage maintained by diode 68 and the voltage desired across potentiometer 62.

When potential is applied to terminal 88, current will flow through resistor and load resistor 86 and through transistor 80, and thence to terminal 90. The junction between resistors 70 and 86 is connected to the emitter of transistor 69. If the current through resistor 70 is sufiiciently small that the potential of the emitter of transistor 69 exceeds the potential of the base, as determined by the setting of the moving contact of potentiometer 62, transistor 69 will tend to become conducting. It will, therefore, cause more current to flow through resistor 76; and this will cause the base of transistor 89 to become more positive and thus will cause transistor to become more conductive, thus tending to increase the current flow through resistor 70. An abnormally large current through resistor 70 will produce the opposites of the eifects described. The values of resistor 70 and 86 and the parameters of transistors 69 and 80 are such that the current flowing through resistor 70 is very largely that which flows through the load resistance 86, so that a reduction in the current drain by transistor 86 through resistor 86 will produce a substantial reduction in the current through resistor 70 and thus will react upon the potential of the emitter of transistor 69. This will produce a corresponding change in the potential of the base of transistor 80. In short, the looped circuit including transistor 69 and 80 is highly degenerative and will tend to maintain the current through load resistor 86 at such a value that the drop across resistor 70 will be an approximately constant value determined chiefiy by the potential applied to the base of transistor 69 by the tap on potentiometer 62. It is, thus, evident that the current supplied to load resistor 86 may be determined by the setting of the movable contact on potentiometer 62. Connecting a high-resistance voltmeter by plug to jack 72, FIG. 1, will permit measurement of the drop across resistor 70 as a guide to such adjustment.

It may be seen from the foregoing discussion that the regulator circuit may be regarded as a two-stage amplifier whose first stage comprises transistor 69 and whose last stage comprises transistor 80. The input to transistor 69 is the variation in potential produced across resistor 70 and the load circuit of transistor 80 may be considered to consist of resistors 86 and 70 in series. It has been found, in agreement with the usual theoretical predictions, that the gain of the successive stages of transistors is approximately 30 for each stage, giving an overall gain for the two stages of about 960. The value of resistor 70 is very much smaller than that of resistor 86; in the embodiment illustrated in the specification, 70 has a value of 241 ohms and resistor 86 may vary from 2,500 to 15,000 ohms or by a ratio of about 6 to 1. It is, therefore, apparent that even the smallest value of resistor 86 is much greater than that of resistor 70, and that the range of variation of resistor 86 is much greater than the change in resistance of the load circuit of transistor 80 produced by the presence (or absence) of resistor 70. The analysis of this circuit on a conventional basis is sufiiciently straightforward so that it is not considered necessary to reproduce it here. A detailed discussion of the properties of feedback amplifiers may be found in the textbook Feedback Amplifiers by H. W. Bode, Bell Telephone Laboratories. However, a somewhat heuristic approach in this case gives a correct description of what has been found experimentally and by more exact analysis, and has the advantage of being quite readily appreciated. Theoretically and with much simplification, a 6 to 1 variation in the value of resistor 86, without regulation, would be expected to cause a 1 to 6 variation in the current through resistor 86. However, the current through resistor 86 also flows through resistor 70 and produces therein a voltage drop. This drop is amplified with a gain of 900 and fed back to resistor 36, in Stabilizing polarity; by this feedback, the effect of variations in resistor 36 is reduced by a factor equal to the reciprocal of the amplifier gain. For the figures given, this means that the variation in current through resistor 256 with variation in the value of 86 will be not 6 to 1 but to 1or in other words the current in resistor 36 will change by about 1 part out of 15 when the lowest value of resistor 36 is multiplied by a factor of 6. Que part out of 15 is equal to about 6%, or to a variation of 13% from an average value. This calculation agrees With experimental results.

It is thus apparent that the particular invention here disclosed, while it affords a range of duration of stimulations which is determined only by the time constants in the amplifier, can provide very accurate control of the duration of the stimulation, by simple accurate control of the duration of the applied carrier signal; and that the intensity of the stimulating current can be controlled with an accuracy that compares very favorably indeed with the stability of voltage provided in residential lighting systems by elaborately instrumented power systems.

FIG. 3 represents an actual physical embodiment employing the circuit represented in FIG. 1. It was rectangular, 5 by 6 by 1.7 em, but the plug 83 extended beyond these dimensions as indicated. Battery 30 was located as indicated in the figure, and its end formed the closure for this part of the case. Switch 82 appears in the representation of this figure simply as an aperture in the case. In order to avoid the protruding knobs, this switch was mounted just below the surface of the case and an opening was left through which a spline shaft could be introduced to rotate the switch to connect the collector of transistor 80 to the selected point of switch 82. This made it possible for the experimenter to change rapidly and conveniently from stimulation over one terminal of plug 83 to stimulation over another terminal. This required, of course, that the experimenter actually approach the animal, but had the advantage that it did not add markedly to the weight, size, or complexity of the equipment.

The problem of maintaining contact with a living animal, particularly if it is able to move freely and engage in all its normal activities, is a very great one. It is undesirable that extraordinarily elaborate methods should be employed which would tend to require any greater disturbance of the animals natural condition than is absolutely essential-in other words, the experimental method should not alter what is to be investigated any more than is unavoidable. For this reason, the use of a current regulator capable of providing the accurate control of the stimulating current is highly beneficial in that even producing physiological changes in the animal which alter the resistivity of its tissues will not alter the stimulating current received by the animal. FIG. 3 represents, primarily by way of completeness, a particular physical embodiment, which was found convenient for use in one application of this invention. It is intended to be connected by a plug to any conventional electrode structure. Obviously there is nothing sacrosanct about the particular case dimensions or form factor chosen, and, indeed, differences in size, physiology and habitat of the experimental animal will require changes in the embodiment. An amphibious animal, for example, would require special precautions to insure that immersion in water would not injure the device or render it inoperative. An attempt to apply a structure such as this to a flying animal would require consideration both of aerodynamic characteristics and desirable weight distribution; and application to an arboreal animal would probably require some consideration to streamlining in order to avoid impediment to the animal in moving among the branches. Solutions to such problems are obviously within the realm of the experi menters technique and knowledge of the art and do not bear upon the basic teachings of my invention.

d TABLE I Resistors Ref.: Value 18 -kilohms 110 26 do 110 28 megohms 2.7 34 kilohms 1 36 do 300 42 "do..-" 44 megohms 2 46 ohms 510 48 ki1ohms 300 52 do 300 54 do 56 62 do 1 64 do 2 66 do 6.2 70 ohms 241 76 kilohms 56 7'8 do 100 Capacitors Ref.: Value 10 microfarad .0001 20 do .0001 22 do .56 38 do .56 50 do- .56

Diodes Ref.: E.I.A. designation 16 IN21 68 IN429 Transistors Ref.: E.I.A. designation 24 2N329 40 2N335 56 2N1234 58 2N498 69 2N1234 2N335 What is claimed is:

1. In combination, a radio receiver means for receiving a carrier-frequency input of a given duration and producing therefrom a signal of duration substantially equal to the duration of the said input; a current regulator means connected to the said receiver means to receive the said signal and responsively thereto and for the duration thereof to produce a current of substantially constant amplitude into a load of varying resistance, and connecting means connected to the said current regulator means to receive the said current for transmission to the said load and connected to the said radio receiver means to transmit thereto carrier-frequency potentials appearing at the said connecting means from the said load.

2. The device claimed in claim 1, further characterized by the fact that the said receiver means and said current regulator means are provided with active devices having power gain, so connected that, in the absence of the said carrier-frequency input, all the said active devices are biased off.

References Cited by the Examiner UNITED STATES PATENTS 2,623,994 12/52 Von Baeyer 325-326 2,652,486 9/53 Guyton 325470 2,703,344 3/55 Anderson 179107 2,800,104 7/57 Cameron et al.

2,992,385 7/61 Lingle 32322 3,038,154 6/62 Zworykin et al 326--16 XR 3,089,998 5/ 63 Reuther 32322 DAVID G. REDINBAUGH, Primary Examiner. 

1. IN COMBINATION, A RADIO RECEIVER MEANS FOR RECEIVING A CARRIER-FREQUENCY INPUT OF A GIVEN DURATION AND PRODUCING THEREFROM A SIGNAL OF DURATION SUBSTANTIALLY EQUAL TO THE DURATION OF THE SAID INPUT; A CURRENT REGULATOR MEANS CONNECTED TO THE SAID RECEIVER MEANS TO RECEIVE THE SAID SIGNAL AND RESPONSIVELY THERETO AND FOR THE DURATION THEREOF TO PRODUCE A CURRENT OF SUBSTANTIALLY CONSTANT AMPLITUDE INTO A LOAD OF VARYING RESISTANCE, AND CONNECTING MEANS CONNECTED TO THE SAID CURRENT REGULATOR MEANS TO RECEIVE THE SAID CURRENT FOR TRANSMISSION TO THE SAID LOAD AND CONNECTED TO THE SAID RADIO RECEIVER MEANS TO TRANSMIT THERETO CARRIER-FREQUENCY POTENTIAL APPEARING AT THE SAID CONNECTING MEANS FROM THE SAID LOAD. 